1. Field of the Invention
The present invention relates to a power module to be used in semiconductor devices for controlling high voltages and large electric current for use in electric vehicles such as electric automobiles and electric trains. In more detail, the present invention relates to a power module substrate having a heat sink for dissipating the heat generated from heating elements such as semiconductor chips.
2. Description of the Related Art
In the conventional power modules as described above, an insulation substrate 2 has been made of a material such as AlN, a circuit layer 6 and a metallic layer 7 have been laminated and bonded on the insulation substrate 2 via a brazing foil, and the metallic layer 7 has been bonded to a heat spreader 8 of a heat sink 3 formed of AlSiC via a first solder layer 5a as shown in FIG. 5. A semiconductor chip 4 is bonded to the circuit layer 6 via a second solder layer 5b, while a water cooled sink 9 is attached to the radiator 8 using external threads 9c (such as pan-head screws). A cooling water flow path 9b for flowing cooling water to flow through is provided within the water cooled sink 9.
Relatively a large amount of heat is generated from the semiconductor chip 4 in the power module substrate having the construction as described above. However, the power module substrates 1 are prevented from being overheated, because the heat generated in the semiconductor chip 4 is transferred to the water cooled sink 9 through the second solder layer 5b, the circuit layer 6, the insulation substrate 2, the metallic layer 7, the first solder layer 5a and the heat spreader 8; cooling water 9a flowing through the cooling water flow path 9b receives the heat and carries it out of the power module substrates 1.
However, it was a problem that the production coast increases in the conventional power module substrates 1 since the large size heat spreader 8 is made of relatively expensive AlSiC.
It was also a problem in the conventional power module substrate 1 that heat cycle service life of the first solder layer 5a is shortened due to difference in deformation of the insulation substrate 2 and the radiator 8 caused by different thermal expansion coefficients between the insulation substrate 2 and the heat spreader 8.
It was an another problem that man-hour for assembling was increased in the conventional power module substrate 1, because the met allic layer 7 should be bonded to the radiator 8 via the first solder layer 5a in a separate process from lamination and bonding of the circuit layer 6 and the metallic layer 7 on the insulation substrate 2.
For solving these problems, a power module substrate 1 as shown in FIG. 6 has been disclosed, wherein the metallic layer 7 is bonded to the heat spreader 8 using the same brazing foil as the brazing foil (not shown) used for lamination and bonding of the circuit layer 6 and the metallic layer 7 on the upper face and lower faces of the insulation substrate 2, respectively.
The problem of shortening the heat-cycle service life can be solved in the power module 1 having the construction as described above, since the first solder layer is not used for bonding the insulation substrate 2 and the heat spreader 8, besides allowing the metallic layer 7 to be bonded to the heat spreader 8 simultaneously with lamination and bonding of the circuit layer 6 and the metallic layer 7 on the insulation substrate 2.
However, manufacturing cost has been increased yet in the improved power module substrate as described above, because the large size radiator is formed using relatively expensive AlSiC.